The liquification of low boiling point gases, such as air and the components of air, such as oxygen, nitrogen and argon, has been practiced for over 100 years, and the liquification of such gases on an industrial scale has been practiced since the beginning of the 20th century. Typically, commercial liquefiers are designed to produce hundreds of tons of liquid cryogens per day. Such industrial liquefiers are reliable, and are capable of producing liquified gas with relatively high energy efficiency. For consumers of liquified gas requiring relatively small quantities, small insulated containers, known as dewars, are filled with liquified gas produced by commercial facilities and transported to the consumer. Consumers of small quantities of liquified gas include hospitals, which require oxygen for delivery to patients and nitrogen for use as a refrigerant. Also, people suffering from chronic respiratory insufficiency that have been prescribed home oxygen by their physicians may have liquified oxygen delivered to their residences.
However, the cost of distributing small quantities of liquified gas is relatively high. In addition, frequent deliveries of liquified gases must be made because of losses due to the eventual warming and boil-off of liquified gas stored in containers. Therefore, there is a need for a liquefier capable of efficiently producing liquified gas at the point of use. For instance, there is a need for a liquefier capable of producing in the range of 0.4 to 5 kilograms per day of liquid oxygen for use in an oxygen patient's residence, or similar amounts of liquid nitrogen for use in physicians' offices or in labs, where it may be used for freezing skin lesions or refrigerating biological samples.
Initially, attempts to provide such a liquefier involved efforts to miniaturize large scale liquefying plants. However, due to the complexity of such systems, which are typically based on the Claude cycle or its variants, these attempts failed. Also, the extremely small mechanical components resulting from the miniaturization of such liquefiers were expensive to produce and unreliable in operation.
In recent years, cryocoolers have been intensively developed. Initially, cryocoolers were developed for the military for use in such applications as cooling infrared sensors, semiconductor chips, microwave electronics, high temperature superconductivity applications, fiber optic amplifiers, etc. The cryocoolers developed for these applications operated in a temperature range of from about 20K to 150K, and their cooling capacity ranged from less than a watt to over 100 watts. For such military applications, the cryocoolers were required to have particular features. For example, in some applications, a fast cool down is important. In other applications, low noise and vibration are desirable. Also, in certain applications, for instance those used in connection with electronic devices, close temperature control of the cooling head is important. Furthermore, certain applications were concerned with preventing frost formation on the insulating envelope and humidity ingress to the cooling components. In addition, the cryocoolers developed for the above-described military applications provided their heat input at or near the lowest temperature point of the cryocooler. For instance, the component to be cooled was typically attached to the cold point (the "cold finger") of the cryocooler, transferring heat directly to that component, with minimal conduction losses. However, for use in small scale gas liquefiers, features such as precise temperature control and quick cool down are not necessary, and serve only to increase the cost of the device. Also, point cooling is inefficient for use in liquefying gases.
With respect to the need for relatively small but steady quantities of oxygen by patients on oxygen therapy, there have been several ways in which the needs of such patients have been met. The most common method for oxygen therapy patients to receive oxygen is through regular deliveries of oxygen produced at a commercial plant. The oxygen may be delivered as either a pressurized gas or as a liquid. When delivered as a pressurized gas, the oxygen presents a hazard because of the high pressure under which it is stored and because oxygen is highly reactive. Oxygen delivered as a liquid is subject to losses resulting from boil-off, which occurs due to the inevitable warming of the liquified gas over time. Because such losses occur even when specially insulated containers, or dewars, are used, deliveries of fresh liquid oxygen must be made on a weekly basis.
It is also known to provide devices which extract or concentrate oxygen found in the ambient air. These devices obviate the need to store a potentially hazardous material. However, these devices are typically not portable, and therefore a person on continuous oxygen therapy must continue to rely on oxygen that has been "bottled" commercially in order to leave their residences. Such reliance has been necessary because, although oxygen concentrators having a production capacity greater than the needs of oxygen patients are known, there has not been an available apparatus and method for producing and storing liquid oxygen in a residence.
For the above-stated reasons, it would be advantageous to provide a method and apparatus for producing and storing relatively small quantities of liquified gas at the location where the liquified gas is to be used. In particular, it would be advantageous to provide a method and apparatus for liquefying oxygen produced in an oxygen therapy patient's residence. In addition, it would be advantageous to provide such a method and device that is economical to operate and reliable.